1. Field of the Invention
The present invention relates to a fuel cell including an electrolyte electrode assembly and a pair of separators sandwiching the electrolyte electrode assembly. The electrolyte electrode assembly includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode.
2. Description of the Related Art
Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (unit cell). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, a predetermined numbers of the unit cells and the separators are stacked together to form a fuel cell stack.
In the fuel cell, an oxygen-containing gas or air is supplied to the cathode. The oxygen in the oxygen-containing gas is ionized at the interface between the cathode and the electrolyte, and the oxygen ions (O2−) move toward the anode through the electrolyte. A fuel gas such as a hydrogen-containing gas or CO is supplied to the anode. Oxygen ions react with the hydrogen in the hydrogen-containing gas to produce water or react with CO to produce CO2. Electrons released in the reaction flow through an external circuit to the cathode, creating a DC electric energy.
In the fuel cell, various proposals for reducing the thickness of the separator and the number of components of the separator have been made. For example, as shown in FIG. 17, according to the disclosure of Japanese Laid-Open Patent Publication No. 2002-75408, a separator 1 of a fuel cell includes a thin sheet of separator body 2, and a large number of first micro-protrusions 3 formed integrally on one surface of the separator body 2, and a large number of second micro-protrusions 4 formed integrally on the other surface of the separator body 2. The first micro-protrusions 3 form a fuel gas flow field 6 between the separator 1 and a fuel electrode 5, and the second micro-protrusions 4 form an oxygen-containing gas flow field 8 between the separator 1 and an air electrode 7.
In Japanese Laid-Open Patent Publication No. 2002-75408, the large number of first protrusions 3 and the large number of second protrusions 4 protrude oppositely from both surfaces of the separator 1. Thus, when a tightening load is applied to the separator 1 and an electrolyte electrode assembly 9 in the stacking direction, the dimensional variations in the height of the first micro-protrusions 3 and the height of the second micro-protrusions 4 in the tightening direction are not absorbed by the separator 1. Therefore, the separator 1 is distorted easily. Consequently, damage or the like may occur in the electrolyte electrode assembly 9 undesirably, and the power generation cannot be carried out efficiently.